X-linked severe combined immunodeficiency (SCID) is one of a number of diseases characterized by profound defects in both T-cell and B-cell immunity. The only treatment for the X-linked form of SCID is bone marrow transplantation, however, the use of somatic cell gene transfer has long been proposed as treatment for SCID and other inherited hematopoietic diseases. A naturally occurring animal model for human X-linked SCID exists in a breeding colony of dogs, which have the same clinical, genetic, immunologic, and pathologic features as human X-linked SCID patients. In humans, this disease is caused by mutations in the gene for the gamma chain of the interleukin-2 receptor (IL2RG). We have recently demonstrated that the canine disease is also caused by a mutation in the IL2RG gene, and thus represents a true homologue of the human disease. The goals of the proposed studies are to use the XSCID dog model to investigate approaches to somatic cell gene therapy for immunodeficiencies. In order to do so, the numbers of affected animals available for study will be increased by breeding affected animals that have been rescued by bone marrow transplantation and assays for canine IL2RG chain function, which can be used to characterize different gene transfer vectors, will be developed. Gene transfer vector viruses containing the canine IL2RG cDNA will be constructed and introduced into X-linked SCID bone marrow and peripheral blood cells, with or without enrichment for hematopoietic stem cells by positive selection with antibodies specific for canine CD34. Transduced cells will be introduced into affected puppies and the ability of these cells to reconstitute the animals' immune systems will be carefully monitored, using sensitive techniques. The canine XSCID model is ideal for performing the controlled experiments, most of which are not possible in human patients, necessary to evaluate the efficacy of gene therapy approaches for the treatment of immunodeficiencies. Since the precise control mechanisms and range of expression of the IL2RG gene remain to be elucidated, as do the effects of excessive or inappropriate expression over the lifetime of an animal, the dog is particularly valuable as a long-lived and relatively large animal, allowing repeated sampling and long-term monitoring of immunologic functions.